An important factor in the present worldwide energy crisis is the cost of energy transportation. In general, energy is more economically transported in the form of nuclear fuel, oil or natural gas than as electricity. In the case of coal, the cost balance is close enough to be tipped either way, depending on circumstances. However, the energy contents of geothermal reservoirs, tides, wind flows, hydraulic heads and gases of low BTU content are definitely not economically transportable over any substantial distance in forms other than as electricity. The latter types of energy sources are being more intensively exploited and it is becoming increasingly important to find ways of decreasing the costs of transmitting electrical power over long distances.
For the high voltages which are most efficient for electrical power transmission, the cost for D. C. transmission lines is considerably lower than for A.C. lines, particularly for underground installations. Also, D.C. systems respond more rapidly to control, avoid the frequency control problems of A.C. systems and offer certain advantages for sea crossings.
Additionally, D.C. is particularly suitable for links between non-synchronous A.C. networks. A further reason to consider D.C. transmission is the increasing need to generate excess power during off-peak periods and store it in an instantly available form for subsequent use during peak demands. Still another consideration is that a great deal of power is consumed as D.C. in end uses such as electrochemical plants, traction devices, elevators and steel mill rolls. Even those uses for which A.C. is preferable could better be served in many cases by individual solid state D.C. to A.C. inverters providing variable frequency control. Thus, it is apparent that there are a number of factors which favor D.C. for power transmission.
Unfortunately, the terminal stations required at the ends of D.C. lines are more expensive than the corresponding ones for A.C. lines. As a rough rule of thumb, the savings in line costs offered by D.C. tend to outweigh the greater terminous costs for lines which are over 300 miles (480 kilometers) long. However, the overall advantage would be more marginal for multi-terminal systems, since terminal costs would constitute a greater proportion of the total costs for such systems. This is a particularly important consideration in view of the advantages of multi-terminal systems (which advantages have been realized for D.C. only at voltages substantially lower than those employed for long distance power transmission).
It is thus apparent that a reduction in terminal costs is essential to fully realizing the inherent advantages of high voltage D.C. (HVDC). A large part of D.C. terminal costs are for A.C. to D.C. and D.C. to A.C. converters (rectifiers and inverters). The most efficient way to supply high voltage D.C. is by rectifying the high voltage output from a step-up transformer operated on relatively low voltage A.C. generated in a conventional power plant. Similarly, almost without exception, the conventional method of utilizing HVDC is to convert it to high voltage A.C. (invert it) and then step the A.C. down to end use voltages through one or more transformers. The latter procedure is followed even when the end use contemplated requires D.C. (end uses for D.C. at typical line voltages are rare or non-existent).
An important reduction in D.C. terminal costs was achieved when it became possible to replace mercury arc rectifier tubes with banks of "high current" thyristors (silicon-controlled rectifiers or SCR's). However, the necessity for employing relatively large numbers of thyristors in series, to accommodate the line voltages involved, results in installations which are still relatively large and expensive. This is well illustrated by the "valves" employed in a long-line, D.C. system which is being developed in the Republic of Zaire to supply power for electrolytic refining of copper.
In the Zaire system, which is representative of the current state of the art, hydropower is used to generate A.C. which is stepped up to 220 kv, rectified to .+-.500 kv D.C., transmitted a thousand miles (1600 kilometers), inverted to A.C., stepped down and again rectified to D.C. for use in the copper mills. The converters at the ends of the line are designed for a normal operating load of 560 megawatts and each includes a total of twelve valves. The valves consist of modularized strings of a large number of thyristors in series and are paired vertically. Each pair, together with the requisite auxiliary equipment, requires a structure 15.3 meters (.about.48 feet) high, 4.1 meters (.about.13 feet) wide and 2.65 meters (.about.8 feet) deep.
The Zaire system is not a multiterminal network and thus does not have to accommodate the switching and control requirements for operating such a network.
In multiterminal A.C. networks, the various terminals can be connected to or disconnected from the line by operation of A.C. circuit breakers capable of handling the volts and amps involved. However, comparable high voltage D.C. circuit breakers are not yet readily available and the conventional approach would be to include at least an inverter circuit, followed by an A.C. breaker at each different terminal, if operation of a really high voltage, multiterminal D.C. system were contemplated.
It has been recognized for some time that development of really efficient high voltage devices for stepping down D.C. from line to terminal voltages ("D.C. transformers") would strongly influence the choice between A.C. and D.C. power distribution systems.
Two different types of D.C. transformers which have received attention in recent years are dynamo-electric rotary transformers and solid state electronic devices which convert D.C. at one voltage to D.C. at a different voltage.
U.S. Pat. No. 3,875,495 (1975) discloses a method of reducing power losses and armature reactions in D.C. dynamos adapted by additional brush sets to function as D.C. transformers. The improvement is said to make practical the use of such transformers to provide smoothly variable voltages for control of D.C. motors and to achieve automatic regenerative braking of the same. It is not proposed to use this type of transformer in power transmission systems and it does not appear that such transformers could be employed at the voltages which would be encountered.
The several known kinds of electronic D.C./D.C. transformers are either "choppers", in which the source voltage is reduced by off/on switching, or are devices which process the power in a D.C. to A.C. (or varying D.C.) to D.C. sequence. Wattage ratings of more than about 1 KW, for either type of device, require the use of thyristors.
The highest wattage rating found in the literature for an individual chopper device is 50 KW for short-term operation and 25 KW for sustained operation: Mormon, Ramsey and Hoff; IEEE Transactions on Industry Applications, Vol. IA-8, No. 5; Sept./Oct. 1972. The preceding ratings are for operation on a supply voltage of 250 VDC.
The highest wattage rating found for a D.C./A.C./D.C. converter is 0.2 MW for a four thyristor, four diode module operating with a current load of 330 amperes and a supply voltage of 750 VDC: Schwarz and Klaassens; IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. IECI-23, No. 2, May 1976. (The 0.2 MW rating is a design rating; the highest wattage attained with an actual module was 10 KW.) Assuming that the 0.2 MW rating could be attained with the described module at the voltages and amperages employed in the above discussed Zaire system, the number of the latter modules required to handle the 560 MW power load in a single terminal in a comparable system would be 560/0.2, or 2800 modules. This would require 11,200 thyristors (and an equal number of diodes). Further, each module would include a saturable core A.C. transformer component.
Thus, it is not apparent from the literature that electronic D.C. transformers offer a practicable alternative to A.C. transformers in HVDC systems.
A third type of "device" which may be described as a D.C. transformer is simply an array of batteries which, in step-down operation, are charged in series across a D.C. power supply and then connected in parallel across a load and discharged. By using several battery sets which are successively cycled between charge and discharge and by appropriately timed switching, an essentially continuous D.C. ouput is obtained.
Since the advent of practical A.C. power systems, around the turn of the century, little or no attention has been paid to battery type transformers. The use of large numbers of batteries for storing off-peak power at power distribution sites has been discussed but supplying power to and retrieving power from the sites at different voltages, or as D.C., has not been suggested.
It is of course evident that power transmitted to a distant terminal could also be battery stored. However, the possibility of using the same batteries to step down the (D.C.) line voltage does not appear to have been considered. This may be attributable to the fact that the voltages employed and the electrical efficiencies realized in the prior art battery type transformers were relatively low.
The state of advancement attained in the battery type transformer art at its apparent prior zenith can be judged from U.S. Pat. No. 443,181 (1890), which is directed to a D.C. distribution system having one or more terminals, each comprising several sets of batteries successively switched back and forth, by a rapidly rotating commutator, between a "high" tension charging circuit and a low tension discharge ("consumption") circuit. The contacts on the commutator "overlap", in the sense that a given set of batteries is not disconnected from a circuit (charge or discharge) until after a second set has been connected in parallel with the first set. (At no time is any set connected in both the charge and discharge circuit).
The following considerations show that if the charging voltage (in the latter system) exceeds a certain value, arcing will occur when the first set of batteries is disconnected from the charging circuit, even though the parallel circuit leg through the second set remains unbroken. The effective voltage across the commutator contacts is the difference between the applied charging voltage and the back emf of the battery (or string) and is equal to the internal IR drop in the battery (or string). If the effective voltage exceeds about 20 volts, arcing will occur as the circuit is broken and the arc will persist until the contacts have been separated a certain distance.
The higher the internal resistance of the battery, the greater the effective voltage across the contacts will be (at a given charging rate). However, even if the resistance is low enough so that the IR drop is only 5% of the applied voltage, the latter voltage cannot exceed about (100/5)20, or 400 volts, if arcing is to be avoided. Furthermore, the only way in which 90% cycle efficiencies can be attained with commercially available batteries is to operate them at impractically low amperage rates. In fact, cycle efficiencies substantially in excess of 80% (IR loss .about.10%) have not been realized in laboratory testing of currently available batteries, according to a recent survey: An Assessment of Energy Storage Systems Suitable for Use by Electric Utilities, Final Report, Vol. II, Electric Power Research Institute Project 225, July 1976.
Thus, the use of battery-type D.C. transformers as a replacement for conventional converters in HVDC terminals is not indicated by the prior art.
It is evident that the prior art does not suggest any practical method of eliminating the need for AC transformers in HVDC systems--particularly in multiterminal systems--and does not contemplate dual function utilization of storage batteries in such systems.